Leopard supports signed applications to improve its security model. All the system utilities, in fact, come signed by Apple. To sign and check applications, the codesign command line utility is available. For example, to display all information about Terminal.app's code signature, open up a terminal and type:

$ codesign -dvvv /Applications/Utilities/Terminal.app

Executable=/Applications/Utilities/Terminal.app/Contents/MacOS/Terminal  
Identifier=com.apple.Terminal  
Format=bundle with Mach-O universal (i386 ppc7400)  
CodeDirectory v=20001 size=2743 flags=0x0(none)   hashes=131+3 location=embedded
Signature size=4064  
Authority=Software Signing  
Authority=Apple Code Signing Certification Authority  
Authority=Apple Root CA  
Info.plist entries=22  
Sealed Resources rules=9 files=63  
Internal requirements count=0 size=12

[robg adds: There's a good amount of information available in the man page, man codesign.]